1. Field of the Invention
The invention relates to the field of bispectral detection using two absorption layers in a single stack in which PN junctions are formed.
2. Description of Related Art
A bispectral detector that is preferred because of its high fill factor and temporal coherence comprises a stack of several semiconductor layers for absorbing different electromagnetic radiation frequency ranges, the layers being insulated from each other and wherein PN junctions are formed in order to collect the charge carriers created by the absorption of incident radiation. Such a detector is described, for instance, in Document U.S. Pat. No. 6,034,407 and the document entitled “Status of HgCdTe bicolor and dual-band infrared array at LETI” by de Destefanis, Journal of Electronic Materials 36(8), p. 1031, (2007). These first two references relate to the material CdHgTe. An example of a similar structure using a different material can be consulted in the following document: Kim, “A Three Wavelength Infrared Focal Plane Array Detector Element”, IEEE Photonics Tech Lett 6(2) p 235 (1994).
In order to better understand the problems encountered with this type of detector, an example of a bispectral array detector 10 is described below, making reference to FIGS. 1 to 3. FIG. 1 is a top view of this detector, represented here in the form of a two-dimensional two-pixel by three-pixel detector, FIG. 2 is a cross-sectional view along line A-A in FIG. 1, and FIG. 3 is a profile showing the cadmium composition x of various alloys of cadmium, mercury and tellurium (CdxHg1-xTe) that form the stack of the detector.
Detector 10 comprises a stack formed by:                a substrate 12 consisting of an alloy of cadmium, zinc and tellurium or “CZT” alloy;        a P-type semiconductor lower absorption layer 14 formed on substrate 12. Layer 14 consists of a CdxHg1-xTe alloy P-doped due to mercury vacancies and a low energy gap. The x14 cadmium composition of layer 14 is selected so that the layer has absorbing properties in a first wavelength range around a wavelength λ14;        an intermediate layer 16 forming a barrier produced on lower layer 14. Layer 16 consists of a material having a high energy gap, for example a CdxHg1-xTe alloy whereof the x16 composition is high in relation to cadmium compositions x14, x18 of layers 14 and 18; and        an upper P-type semiconductor absorption layer 18 formed on layer 16 that forms a barrier. Layer 18 consists of a CdxHg1-xTe alloy P-doped due to mercury vacancies and a low energy gap. The x18 cadmium composition of layer 18 is selected so that the layer has absorbing properties in a second wavelength range around a wavelength λ18 such as λ18<λ14.        
Type N semiconductor zones 20 are also produced in upper layer 18, for example by boron ion implantation. This ion implantation step has the effect of converting the P-type intrinsic doping to N-type doping and thus forms an array of PN junctions and hence photodiodes.
Openings 22 are also machined through upper layer 18 and intermediate layer 16 as far as lower layer 14 in order to obtain access to the latter. N-type semiconductor zones 24 are made in lower layer 14 by applying N-doping by boron ion implantation, for example, to those parts of lower layer 14 that just touch the bottom of openings 22. An array of PN junctions, and hence photodiodes, is thus formed in lower layer 14.
Semiconductor zones 20 and semiconductor zones 24 preferably form detection arrays of L rows by C columns respectively where L and C equal 2 and 3 respectively in the example shown and the array of zones 24 is offset relative to the array of zones 20 so as to obtain a zone 24 in the centre of a rectangle or a square consisting of four zones 24.
A passivation layer 26 (not shown in FIG. 1 for the sake of clarity) produced with the aid of a CdTe/ZnS bilayer is also deposited on the exposed face of upper layer 18 and in openings 22.
Finally, a metallic contact pad 28 is formed on upper layer 18 above each zone 20 and penetrates into zone 20 in order to collect the charges contained therein. Similarly, a metallic contact pad 30 is deposited in each opening 22 in the form of a layer that covers the flanks of the opening and penetrates into corresponding zone 24 in order to collect the charges contained in that zone. Contact pad 30 extends on the upper face of passivation layer 26 in order to facilitate connection of pad 30 to external interfacing (not shown). Finally, an indium bump 32, 34 is used on that part of each contact pad 28, 30 formed on the upper face of passivation layer 26 in order to hybridize the stack on a readout circuit (not shown) by using flip chip technology.
Detector 10 described above is a backside illuminated sensor. The exposed face of substrate 12 receives electromagnetic radiation RE which penetrates the stack. The portion of radiation RE contained in the first wavelength range is absorbed by lower layer 14, and the portion of radiation RE contained in the second wavelength range is absorbed by upper layer 18.
As is known in itself, the absorption of photons in lower layer 14 and upper layer 18 releases charge carriers that diffuse into semiconductor zones 20, 24 and are collected via contact pads 28, 30. A bias voltage is or is not applied between a common peripheral contact (not shown in FIG. 2) and contact pads 28, 30 in a manner that is known in itself.
The role of intermediate layer 16 is to prevent the charge carriers created in one of layers 14, 18 from diffusing into the other layer 14, 18, thereby producing a phenomenon known as crosstalk which is detrimental to detection quality. This function is more commonly referred to as a “barrier”.
The quality of the barrier function of intermediate layer 16 depends mainly on the difference between the band gap value of intermediate layer 16 on the one hand and that of lower layer 14, and that of upper layer 18 on the other hand. Intermediate layer 16 forms a potential barrier that separates the valence and conduction bands of lower layer 14 and upper layer 18, thus limiting the movement of charge carriers from one layer to another.
In a CdxHg1-xTe type semiconductor alloy, the energy gap value is chiefly determined by the mercury composition (1−x) or, equivalently, by the cadmium composition x. FIG. 3 illustrates a typical profile for the cadmium compositions x of the various layers of the stack with intermediate layer 16 having an x16 composition that is preferably at least 50% higher than each of the x14, x18 compositions of lower layer 14 and upper layer 18.
Openings 22 are necessary in order to access lower layer 14, thus making it possible to produce semiconductor zones 24 and produce contact pads 30 to collect the charges in zones 24.
However, regardless of the machining technology used to form openings 22, the walls of the openings always have many imperfections. The crystal quality of the flanks of the hole may be degraded by etching with this resulting in a high surface recombination rate. In fact, those parts of openings 22 that are present in upper layer 18 are recombination sources for charge carriers in that layer as well as sources of low-frequency noise associated with this generation current for the PN junctions of this layer 18.
One solution that is often adopted in order to mitigate this problem is to move the PN junctions of upper layer 18 away from openings 22. This therefore reduces the pixel fill factor.
Another solution involves using plasma machining techniques to produce openings 22 and then repairing the flanks of the openings by annealing. This solution would be the ideal preference but requires perfect mastery of the etching process which entails onerous technological development work.